The present invention relates to surface inspection systems and methods and, more particularly, to the inspection of articles or workpieces, such as silicon wafers, to detect the presence of defects such as particles or pits on the surface and to distinguish therebetween.
Surface inspection systems are commonly used for the inspection of articles or workpieces such as silicon wafers to detect the presence of defects on the wafer surface. When the inspection indicates a large number of defects, the wafer may be sent back for recleaning. If the defects are particles or other debris on the wafer surface, the recleaning is successful. However, if the defects are pits or xe2x80x9cCOPSxe2x80x9d (crystal originated pits) in the wafer surface, they are not removed by recleaning. Because such surface inspection systems fail to distinguish between pit defects and particle defects, the wafer is typically sent back for recleaning regardless of whether the defects are pits or particles. Because these defects may be pits, recleaning the wafer may result in nothing more than a waste of time and resources. It would be advantageous to be able to distinguish pits in the surface of the wafer from particles located thereon.
The present invention provides a surface inspection system and method which not only detects defects such as particles or pits on the surface of a workpiece, such as a silicon wafer, but also distinguishes between pit defects and particle defects. This makes it possible to easily ascertain whether the workpiece requires recleaning to remove particle defects, or whether other measures must be taken.
In a broad aspect, the surface inspection system comprises an inspection station for receiving a workpiece and a scanner positioned and arranged to scan a surface of the workpiece at said inspection station. The scanner includes a light source arranged to project a beam of P-polarized light and a scanner positioned to scan the P-polarized light beam across the surface of the workpiece. The system further provides for detecting differences in the angular distribution of the light scattered from the workpiece and for distinguishing particle defects from pit defects based upon these differences. The differences in the angular distribution of the scattered light may, for example, be detected by comparing the amount of light scattered in a direction substantially perpendicular from the surface of the workpiece to the amount of light backscattered from the surface of the workpiece. The detection of differences in the angular distribution of the scattered light may also, for example, involve identifying a dip in the intensity distribution of the scattered light.
The scanner preferably scans across the surface of the workpiece along a relatively narrow scan path during rotational and translational travel of the workpiece. More specifically, the system preferably has a transporter arranged for transporting the workpiece along a material path and a rotator associated with the transporter and arranged for rotating the workpiece during translational travel along the material path. The scanner is positioned and arranged for scanning a surface of a workpiece during rotational and translational travel along the material path so that the entire surface of the workpiece is raster scanned in a spiral pattern. The scanner includes either a P-polarized light source or a light source coupled with a P-polarized filter positionally aligned with the light source.
A collector also is arranged for collecting light reflected and scattered from the surface of the workpiece during rotational and translational travel along the material path. The collector includes a dark channel detector positioned for detecting light which is scattered from the surface of a workpiece. The dark channel detector includes a plurality of collectors positioned and arranged for collecting light at different angles relative to the surface of the workpiece. Each collector includes a photodetector for generating electrical signals in response to the collected light. The electrical signals from photodetectors located at the different angles are compared to determine the differences in angular distribution of the scattered light.
The plurality of collectors preferably includes a forward channel collector arranged to collect light components scattered forwardly from the surface of the workpiece at a relatively small angle with respect to the specular reflection from the workpiece, a center channel collector positioned adjacent to the forward channel collector and arranged to collect light components scattered substantially normal from the surface of the workpiece at a relatively medium angle, and a back channel collector positioned adjacent to the center channel collector and arranged to collect light components scattered backwardly from the surface of the workpiece at a relatively large angle.
When the scanned light beam contacts a defect, such as a pit or a particle, light is scattered from the surface and is collected by the collectors. The intensity of the scattered light, and the time of its detection during the scan, provide information about the size and location of the defect on the surface of the workpiece. Furthermore, the nature of the defect, i.e. whether it is a pit or a particle, can be ascertained by detecting differences in the angular distribution of the light scattered from the workpiece. For example, if the defect is a pit, the amount of light scattered and detected by the center channel collector is typically greater than that detected by the back channel collector. Alternatively, if the defect is a particle, the amount of the light detected by the center channel collector is typically less than that detected by the back channel collector and/or the forward channel collector. The dark channel collector system provides the surface inspection system of the present invention with high sensitivity to more readily identify, classify, and/or provide a topography of the condition of the surface of an article or a workpiece, including defects such as particles, pits and the like, in and on the surface of a workpiece.
According to one specific embodiment of the invention, a P-polarized light beam is directed along a predetermined relatively narrow scan path and at a relatively low angle of incidence with respect to the surface of the workpiece. The method preferably also includes imparting a rotational and translational movement of the workpiece during the narrow scan so that the narrow scan path traverses the entire surface of the workpiece along a spiral path.
The surface inspection system and method of the present invention advantageously distinguish pits in the surface of the wafer from particles on the surface of the wafer and therefore determine whether cleaning or some other course of action, e.g., altering the conditions of manufacture and storage, can be used to cure the defects. In addition, the surface inspection system and method provide high spatial resolution, a small field of view at the object plane which, in turn, provides improved edge detection performance, improved repeatability in the inspection process and reduces interference signals caused by scatter from air molecules.